Tom Lane wrote:
>A two-column index cannot support a search based on only the second
>column. It can support a search based on only the first column,
>however. (Think about the physical index ordering to see why.)
>
I see, I'll declare a seperate index on the second column if the need
arises.
>
>Presently, EXPLAIN doesn't show you exactly what the indexscan
>conditions are, so it's not obvious what the difference is between
>your first three cases. You can figure it out if you have the patience
>to study EXPLAIN VERBOSE output, but that's pretty ugly :-(. I have
>been thinking about whether it wouldn't be possible for EXPLAIN to emit
>a pretty-printed version of the scan conditions. In that case you could
>see what was happening in examples like this.
>
To be honest I've never understood too much of the EXPLAIN output to
begin with, so I'll probably get in way over my head when I try and
understand the VERBOSE output. (Chances are I'll get it all backwards
and degrade performance instead :-( ).
Is there any recommended reading on understanding EXPLAIN/ performance
in general? There's probably more to be said on the subject than what's
in the Users Guide.
Thanks for the quick responses, mostly I just read the list and I am
impressed how fast and (mostly) on target the advice on this list often is.
( Thought I'd do some kissing up, I might have a couple more questions
over the next few days ;-)
Regards,
Rob
